Process for producing potassium peroxydiphosphate

ABSTRACT

Potassium peroxydiphosphate is produced by electrolyzing an anolyte containing an aqueous mixture of potassium, phosphate and fluoride ions and catholyte containing an aqueous mixture of phosphate ions, said anolyte and catholyte being separated by diaphragm means; peroxydiphosphate values are produced in the anolyte at a pH of from about 9.7 to 14.5, and these values are prevented from substantial migration to the catholyte by said diaphragm.

United States Patent Paul R. Mucenieks Trenton, NJ. 688,525

Dec. 6, 1967 Oct. 26, 1971 FMC Corporation New York, N .Y.

lnventor Appl. No. Filed Patented Assignee PROCESS FOR PRODUCING POTASSIUM PEROXYDIPHOSPHATE 10 Claims, 2 Drawing Figs.

US. Cl

Int. Cl Field ofSearch 204/83, 82,

References Cited UNITED STATES PATENTS 6/1957 Muller 204/82 2,772,229 11/1956 Karr OTHER REFERENCES Fichter et al., Helv. Chim. Acta 11, 323- 337 (1928). Lowry, Inorganic Chemistry, London, 1931. pp. 495- 6 Primary Examiner- Patrick P. Garvin Attorneys-Eugene G. Seems, Frank lanno and Milton Zucker PATENTEDUBT 2 6 IQYI SHEET 1.0? 2

FIG.

KOH H3PO4 H2O lllli E D O H M c E D O N A 'COMPART- j I MENT COMPART- MENT PRODUCT PAUL R. M UCENIEKS M440 Q/l (AV PERCENT CURRENT EFFICIENCY PATENTEDUCT 2s IHTI 3 6 l 6 325 SHEET 2 BF 2 FIG. 2

PERCENT CONVERSION 10-20 PERCENT CONVERSION 20-40 PERCENT CONVERSION 4o-so PERCENT CONVERSION 60-80 1 l l l l 1 1 O 8 9 I0 I I l2 l3 l4 l5 ANOLYTE PH PAUL R. MUCENIEKS l/\/'/ ulz VZM pm, IN W PROCESS FOR PRODUCING POTASSIUM PEROXYDIPIIOSPI'IATE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the production of potassium peroxydiphosphate by electrolysis of a potassium phosphate solutron.

2. Description of the Prior Art The preparation of potassium peroxydiphosphate in the laboratory by electrolytic methods is known. Prior workers have reported the production of potassium peroxydiphosphate by placing a solution of potassium phosphate, potassium fluoride, and potassium chromate (K,Cr0,) in a platinum crucible, inserting a platinum tube or wire into the solution and electrolyzing the solution using the platinum crucible as the anode and the tube or wire as the cathode. This is reported by F. Fichter and E. Gutzwiller, Helv. Chim. Acta 11, 323-337 (I928). This process is applicable to the formation of only laboratory amounts of potassium peroxydiphosphate because of the inability to scale such a process up to commercial production. The resulting product is reported as being very hygroscopic and not subject to air drying. Moreover, prior workers have not been able to remove chromium-containing impurities from the finished product, and this is desirable because these impurities act as a decomposition catalyst for the peroxydiphosphate. In addition, only low anode current densities can be used, otherwise the yield drops below 20 percent. Further, temperatures below l5 C. are normally required and potassium peroxymonophosphate, which is produced as a byproduct, must be removed periodically in order to obtain the potassium peroxydiphosphate.

As a result, it has been impossible to produce potassium peroxydiphosphate on a commercial basis because of the lack of an effective process which is efficient, cheap and yields high-purity potassium peroxydiphosphate as a product.

OBJECTS OF THE INVENTION It is an object of the present invention to produce potassium peroxydiphosphate in good yields and at commercially acceptable efficiencies.

It is a further object to produce potassium peroxydiphosphate by a process which yields a pure product and which can be carried out commercially in batch form or in continuous production.

These and other objects will be apparent from the following description of the invention.

SUMMARY OF THE INVENTION 1 have found that potassium peroxydiphosphate can be produced in good yields by introducing an aqueous mixture containing potassium, phosphate and fluoride ions into the anode compartment of an electrolytic cell as the anolyte, introducing an aqueous mixture containing phosphate ions into the cathode compartment of said electrolytic cell as the catholyte, the catholyte and anolyte of said electrolytic cell being contained by means (preferably diaphragm means) which permit potassium and/or phosphate ions to pass freely between the anolyte and catholyte but which prevent any substantial amounts of peroxydiphosphate values in the anolyte from mixing with the catholyte, passing an electric current through said catholyte and anolyte by means of a cathode in said catholyte and an anode in said anolyte, converting phosphate values to peroxydiphosphate values in said anolyte having a pH of about 9.7 to about 14.5, removing an anolyte enriched in peroxydiphosphate values from said electrolytic cell, precipitating potassium peroxydiphosphate values from the peroxydiphosphate-enriched anolyte and separating and recovering said potassium peroxydiphosphate values from the anolyte mother liquor.

I have found further, that the above process can be carried out on a continuous bases by recycling said anolyte mother liquor to said anode compartment along with the removed catholyte solution and introducing a fresh, aqueous solution containing phosphate ions (and preferably containing both potassium and phosphate ions in a K/PO. mole ratio of 0.5-3:l into said cathode compartment as an electrolyte, and continuing the electrolysis.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, FIG. I is a diagrammatical flow sheet of the process and an electrolytic cell for carrying out the electrolysis; FIG. 2 is a graphic illustration of the relationship between the pH of the anolyte and current efficiencies of the electrolytic cell at different levels of conversion.

DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS In carrying out the present invention an aqueous anolyte solution is prepared preferably by mixing together water, potassium hydroxide, phosphoric acid and hydrofluoric acid. The anolyte should have a phosphate ion concentration of from about 1 to about 4 molar and preferably from about 2 I-IPO, 3 molar; the fluoride ion should be present in amounts of about 0.5 to 1.25 atoms of fluoride ion for each phosphate ion. In general, fluoride ion concentrations of 0.1 to 5 molar (and preferably 1 to 3.6 molar) may be used. The anolyte solution should contain a potassium ion for every fluoride ion present in the anolyte solution, and further, additional potassium ions in the amounts of two to three ions (preferably 2.5 to 2.8 atoms) should be present for each phosphate ion in solution. It is preferred to make up the anolyte solution by mixing KOH, H,PO, and HP in an aqueous solution because these materials can be obtained in a pure state, and the resulting anolyte solution contains a minimum of impurities. However, an anolyte can also be prepared from KH,PO,, K,HPO,, K,PO,, KHF}, KF, KOl-I and all hydrates of the above salts, notwithstanding the generally higher cost and higher impurity level compared with the preferred ingredients.

The aqueous solution thus prepared is passed into the anode section of an electrolytic cell and used as the anolyte. The cell also contains a solution in the cathode section which serves as the catholyte. The catholyte is an aqueous solution containing phosphate ions obtained, e.g., by dissolving a phosphoric acid in water, and preferably contains both potassium and phosphate ions in a K/PO, mole ratio of 0.53: 1. The optimum balance of K and PO, values are obtained in the cell when the K/PO, mole ratio is 2:1. The catholyte is prepared preferably by mixing together potassium hydroxide and phosphoric acid. The concentration of the catholyte solution is adjusted to prescribed limits as set forth hereinafter to permit continuous, recycle operation. For reasons of purity and cost, it is preferred to prepare the catholyte from potassium hydroxide and phosphoric acid although potassium phosphate salts such as KI-I PO K i-IP0 and hydrates of these salts can be used.

The anolyte and catholyte solutions of the electrolytic cell must be contained in a manner to prevent any substantial amounts of peroxydiphosphate values which are formed in the anolyte from mixing with the catholyte, but without preventing the flow of an electric current between the solutions. It is preferred that the anolyte and catholyte be separated by a diaphragm which permits the flow of an electric current between the anolyte and catholyte but which prevents substantial amounts of peroxydiphosphate values in the anolyte from diffusing into the catholyte. The flow of current between the anolyte and catholyte takes place by passing certain ions, principally potassium and/or phosphate ions, through the diaphragm and is a necessary part of the electrolytic circuit of the cell. The diaphragm in the electrolytic cell can be any porous membrance such as a porous, porcelain sheet, asbestos membrance, plastic membrane, or a cation exchange membrane, etc. In the case of a cation exchange membrane, this will permit the passage of potassium ions from the catholyte to the anolyte selectively without pennitting anions, such as peroxydiphosphate or phosphate anions, from passing through the cation exchange membrane.

An electrode then is immersed in each catholyte and anolyte solution. These electrodes can be any material which can conduct an electric current and which does not react with the solutions of the cell during electrolysis. Generally, noble metals such as platinum or gold or tantalum-reinforced platinum are preferred as the anodes. The cathodes can be made of lead, graphite, stainless steel or of noble metals such as platinum or gold. The cathodes can be in the form of plates, wire screens or in the form of spirals or other configurations made of a hollow tubing. If the cathode is in the form of a hollow tubing, a cooling solution can be passed through the tube and serve to cool the catholyte. An electric potential is placed across the electrodes (the anode and cathode) by means of a battery, rectifier, or other source of direct current to complete the electrolytic cell.

The electric potential which is applied across the electrodes must be sufiicient to cause a positive electric current to flow outside the cell, from cathode to anode, so that hydrogen gas is generated at the cathode, and phosphate values are converted to peroxydiphosphate values at the anode. Normally an EMF of at least about 2.0 volts has been found operable with from about 3 to 8 volts being preferred. The electrolytic conversion of phosphates to peroxydiphosphates may be conducted at any temperature of from 10 to 90 C. with temperatures of from about 10 to about 70 C. being preferred. The

. preferred anode current density which is used in operating the cell may range from 0.1 to l aJcm. of anode surface, with 0.3 a.,'cm. of anode surface being optimum.

In carrying out the present process it is necessary to control the pH of the anolyte to obtain maximum conversion of phosphate ions to peroxydiphosphate ions at high current efficiencies. The current efficiency is determined by comparing the amount of peroxydiphosphate values formed by a unit quantity of electricity with the theoretical amount of peroxydiphosphate which that amount of electrical energy can produce. The current efficiency is a separate and distinct measurement from the degree of conversion, in that the latter expresses only the percent of phosphate ions convened to peroxydiphosphate ions, regardless of the quantity of electricity used to effect the conversion.

In general, the anolyte should be maintained at a pH of from about 9.7 to about 14.5 to obtain conversions (phosphate ions to peroxydiphosphate ions) greater than percent at current efficiencies of at least 50 percent. Highest conversions are obtained when the pH of the anolyte is from about 11.8 to about 13.5 with the optimum being obtained at a pH of about 12.2 to 12.6. The pH of the anolyte can be controlled most readily by regulating the concentration of potassium hydroxide in the anolyte solution.

During electrolysis, the pH of the anolyte changes and becomes progressively less alkaline. Accordingly, optimum conditions for obtaining maximum conversion can be obtained by constantly adjusting the pH of the anolyte in the electrolytic cell by the addition of KOH or-by commencing operation on the alkaline side of the preferred range and continuing electrolysis until the anolyte has reached the lowest pH at which operation is desired.

The reactions that presumably occur at each of the halfcells are as follows:

At the cathode:

At the anode:

21 0 2 electrons l,0

The desired overall re action of the electrolytic cell is:

in the above description of the electrolytic cell, phosphate ions are converted to peroxydiphosphate ions at the anode, while i-l ions are converted to hydrogen gas at the cathode. The electronic flow necessary to carry out these conversions within the cell is obtained by the transfer of ions through the llll diaphragm. For example, potassium ions may flow from the anode compartment through the diaphragm into the cathode compartment, and/or P0,, l-lPO,= or l-LJO, ions may flow from the cathode compartment through the diaphragm to the anode compartment. When substantial potassium ion transfer occurs, the anolyte, depleted of its K values, becomes progressively less alkaline; simultaneously the catholyte becomes more alkaline as its K concentration increases. In addition to the transfer of ions through the diaphragm, some catholyte solution may be permitted to flow through the diaphragm into the anolyte. However, it is not desirable to have any substantial flow of anolyte through the diaphragm into the catholyte because of the possible transfer and loss of peroxydiphosphate values.

The electrolysis is continued until the desired conversion of phosphate to peroxydiphosphate ions has been obtained. The exact amount of conversion desired will depend upon the minimum current efficiencies which can be tolerated, regardless of whether the system is operated as a batch process or a continuous process; current efficiencies normally decrease as higher conversions are obtained. When the desired peroxydiphosphate concentration has been obtained, the anolyte is removed from the anode compartment of the cell and placed in an evaporator-crystallizer unit. in this unit, water is evaporated from the solution, and potassium peroxydiphosphate is crystallized and separated from the mother liquor.

When the anolyte is removed from the electrolytic cell and passed into the evaporator-crystallizer, the pH of the anolyte should be adjusted to between 11.8 and 12.6, and preferably 12 to 12.6. This can be done by the addition of KOH if the solution has a pH below 1 1.8, or by the addition of phosphoric acid, if the solution has a pH above 12.6. Obviously, other acids or bases can be used to achieve the same results, but the above additives are desired since they do not introduce foreign ions into the system. The pH of the anolyte is adjusted between 1 1.8 and 12.6 before evaporation of water and crystallization of the potassium peroxydiphosphate product in order to facilitate separation of the potassium peroxydiphosphate product from the mother liquor. If the anolyte has a pH outside of this range, the resulting potassium peroxydiphosphate crystals retain excess amounts of water and become extremely difficult to filter or otherwise separate from the resulting mother liquor. When the anolyte is maintained at optimum pH conditions for conversion of the phosphate to peroxydiphosphate, namely a pH of from about 12 to 12.6 by the constant addition of KOH into the anolyte during the electrolysis, the anolyte which is removed from another compartment of the electrolytic cell will be at the proper pH for immediate evaporation and concentration to recover the potassium peroxydiphosphate product.

The mother liquor containing some unconverted phosphate values, potassium values and fluoride values is then mixed with the exit catholyte solution, and the mixture is recycled to the anode compartment of the electrolytic cell. ln this way, succeeding anolytes are made up from the mother liquor obtained after crystallizing potassium peroxydiphosphate from Lthe anolyte and the exit catholyte solution. A fresh catolyte solution is then made up, preferably in the form of an aqueous, K l-llO solution. The fresh catholyte is made up preferably by mixing together potassium hydroxide and phosphoric acid. in general, the concentration of the fresh catholyte is adjusted so that the outgoing catholyte from the electrolytic cell is at the proper concentration, when mixed with the mother liquor from the anolyte, to provide a suitable anolyte as previously defined. Providing a phosphate ion containing solution as the catholyte serves several purposes. Initially, it serves as a method of collecting and recycling any and all potassium values which migrate into the catholyte from the anolyte during electrolysis. Additionally, the presence of phosphate ions in the cathode compartment provides the necessary conductivity of the catholyte. Finally, the use of phosphate ions (preferably with potassium ions in a K/PO, mole ratio of 0.5-3:l) in the catholyte prevents any foreign ions from diffusing into the anolyte solution and contaminating or destroying peroxydiphosphate values.

The above electrolysis reaction can be conducted batchwise or continuously. In a continuous operation, electrolytic cells should be used in which the separate electrolytes are introduced at one end of the cell and flow along opposite sides of the membranes until they are removed from the cell at the opposite end. During this flow the concentration of the peroxydiphosphate ion in the anolyte increases. Thereafter the peroxydiphosphate values are recovered and the remainder of the anolyte and the exit catholyte are recycled to the anode compartment along with any makeup chemicals required to obtain a desired anolyte composition. The solutions can be introduced in other such cells in cascade, if desired, so that instead of a single cell, a plurality of electrolytic cells can be used.

The present invention will now be described by reference to the drawings in which FIG. .1 is a diagrammatical flow sheet of one embodiment of the process, and FIG. 2 shows, in graphic form, the current efficiencies as a function of the pH of the anolyte at different levels of conversion.

In FIG. 1 of the drawings, the anolyte solution enters the electrolytic cell 2 through line 4. The electrolytic cell 2 is made up of anode compartment 6 and a cathode compartment 8, separated by diaphragm 10. In the anode compartment 6 an anode 12 is connected by electric battery 14 to the cathode 16 in the cathode compartment 8. The catholyte is made up preferably by mixing potassium hydroxide, phosphoric acid and water in catholyte makeup tank 18, and the resulting aqueous, potassium phosphate solution is passed through line 20 into the cathode compartment 8. The electrolytic cell 2 is then energized by connecting anode l2 and cathode 16 to battery 14. In the anode compartment 6 phosphate ions are converted to peroxydiphosphate ions at the anode 12. The electric current is carried between the anode and cathode compartments principally by the transfer of potassium ions along with some hydrogen ions through the diaphragm from anode compartment 6 to cathode compartment 8. As this occurs the anolyte becomes less basic, and the pH decreases. If the anolyte is at the optimum pH of 12 to 13.3 when electrolysis commences, the pH of the anolyte can be maintained within this range by the addition of potassium hydroxide through line 22 and line 24 directly into the anode compartment 6 of the electrolytic cell 2. If direct introduction of KOH into the anode compartment is not desired, the pH of the anolyte which enters the anode compartment 6 can be maintained at initially high alkaline pH, e.g., about 13 to 14, and the pH allowed to drop, with continued electrolysis to a pH of not lower than about 9.7. In this way, electrolysis of the anolyte through the optimum pH range will take place during a large portion of the electrolysis.

In the cathode compartment 8 hydrogen ions are converted at the cathode 16 to hydrogen gas which is evolved from the cathode compartment 8. The potassium ions (and some hydrogen ions) which carry the electric current in the electrolytic cell and pass from the anode compartment 6 through the diaphragm 10 into the cathode compartment 8 enrich the potassium content of the catholyte. At the same time the catholyte increases in pH because of the loss of H ions. Electrolysis is continued until the desired conversion of phosphate ions to peroxydiphosphate ions in anode chamber 6 has been obtained commensurate with permissible current efficiencies. When this occurs the anolyte is removed from the anode compartment 6 through line 26 and is treated to adjust the pH between 11.8 and 12.6 and preferably between 12 and 12.6. This is most conveniently doneby adding KOH from line 22 and line 22 directly into the withdrawn aholyte solutionfThe withdrawn anolyte solution from line 26, whose pH has been adjusted between 1 1.8 and 12.6, is then placed in the evaporator-crystallizer 30 where a portion of the water in the anolyte is evaporated. When sufficient amounts of the water have been evaporated, potassium peroxydiphosphate crystallizes from the evaporated solution and is separated from the mother liquor. The potassium peroxydiphophate removed from the evaporator-crystallizer 30 through line 32 and recovered as product, while the mother liquid is separated through line 34. The catholyte is also removed from cathode compartment 8 after the electrolysis has been completed through line 36. The exit catholyte can be mixed with the anolyte through line 38 prior to evaporation and crystallization of the potassium peroxydiphosphate product, if desired. Alternately, the exit catholyte can be mixed with the mother liquor from the evaporator-crystallizer 30 by passing the catholyte through lines 36 and 40 until it mixes with the mother liquor in line 34. The mixture of catholyte and mother liquor is sent through line 4 back to the anode compartment 6 as makeup anolyte. The pH of the anolyte in line 4 can be controlled by adding KOH through lines 22 and lines 44 to give the desired pH range, normally between 9.7 and 14.3.Also, provision is made for the addition of HF through line 46 into the anolyte makeup solution in line 4 to replace any fluoride ion lost in processing. Fresh catholyte solution from makeup tank 18 is then passed into the cathode compartment 8 through line 20 to complete the electrolytic cell for continued production of potassium peroxydiphosphate.

In FIG. 2, there is shown, in graphic form, the relationship between current efficiencies and the pH of the anolyte at different levels of conversion. The uppennost curve illustrates that to obtain current efficiencies of about 50 percent or above, when the degree of conversion of phosphate ions to peroxydiphosphate ions is no more than 20 percent, the pH of the anolyte must be between the range of about 9.7 to about 14.3. The next curve adjacent the uppermost curve illustrates that to obtain current efficiencies of 50 percent or above, when the degree of conversion is from 20 to 40 percent, the pH of the anolyte must be between about 10.7 and about 14.2. The remaining curves show the relationship between anolyte pH and current efficiencies at levels up to about percent conversion of phosphate ions to peroxydiphosphate ions. As will be observed from FIG. 2, the maximum current efficiencies, at any conversion level is obtained when the pH of the anolyte is about 12.5.

FIG. 2 also illustrates that current efficiencies decrease with increased conversion of the phosphate ions to peroxydiphosphate ions, regardless of the pH value of the anolyte. This decrease in current efficiency is due to the use of increasingly larger portions of the electronic current to electrolyze undersired side reactions rather than the principal conversion of phosphate ions to peroxydiphosphate ions.

The following examples are given to illustrate the present invention and are not deemed to be limited thereof.

EXAMPLE 1 A two-compartment electrolytic cell was made up of whose dimensions were 10 X12 X20 cm. and which was equipped with a platinum mesh anode having a surface area of 14 cm. and a coiled, stainless steel (S5304 tubular cathode having a surface area of 340 cm. A porous, ceramic plate served as the diaphragm between the cathode compartment and anode compartment and had a surface area of 57 cm. A tubular glass coil was placed in the anode compartment and was connected by a hollow rubber tubing to the stainless steel tube serving as the cathode. Water was passed through the glass coil in the anode compartment, through the rubber tubing connection and finally through the steel tubing cathode in the cathode compartment to maintain the temperature of the cell constant during electrolysis.

One liter of an anolyte solution was made up containing 2 moles of phosphoric acid, 2.4 moles of HF and 8.0 moles KOH. The catholyte solution had a volume of 0.5 liters and contained 0.96 mole phosphoric acid and 1.19 moles KOI-l. The catholyte and anolyte were then added to their respective compartments in the cell and the solutions electrolyzed with a current of 4.2 amperes. The current density of 0.3 a./cm. at the anode, 0.012 a./cm. at the cathode and 0.075 a./cm. at the diaphragm. Electrolysis was continued until 60 percent of the phosphate present in the anolyte was convened to peroxydiphosphate. This took 9 hours and 34 minutes, The original anolyte solution which had a pH of about 12 was maintained at this pH by the addition of 0.54 mole of KOl-l during the electrolysis. After the electrolysis was complete, the anolyte was removed from the electrolytic cell and evaporated in a ucuum-type crystallizer until 0.48 mole (166g.) of K, P, 08 was precipitated and recovered. The mother liquor recovered after evaporation of the anolyte solution was mixed with the exit catholyte and sufficient water to make 1 liter; this mixture was placed in the anode compartment of the electrolytic cell. Five hundred milliliters of fresh catholyte solution was made up containing 0.96 mole P0, and 1.49 moles of KOH. Electrolysis of the cell was then resumed and was carried out as set forth previously until 60 percent of the phosphate ions present were converted to peroxydiphosphate ions. This toolt 7 and 40 minutes. During the electrolysis the pH of the anolyte which was at about 12 during the commencement of the electrolysis was maintained at about 12 by the addition of 0.43 mole of KOH into the anolyte as electrolysis progressed. The anolyte solution, having a pH of from 12 to 12.6, was removed from the cell, partially evaporated, and 166 g. of potassium peroxydiphosphate was precipitated and recovered. The resulting product contained 93 percent potassium peroxydiphosphate and 1 percent KF. The product was further purified by recyrstallization from water, and the resulting potassium peroxydiphosphate was found, on analysis, to be 99+ percent pure, with no detectable fluoride or chromium content. it was a dry, white, free-flowing, stable, crystalline product and was not hygroscopic.

EXAMPLE 2 Four electrolytic cells, each having substantially the same structure described in example 1, were connected together in series so that the catholyte and anolyte of each cell flowed into the cathode and anode chamber of the succeeding electrolytic cells continuously. An anolyte solution was then made up containing 2.4 moles HP, 2.0 moles l l PO and 8.0 moles KOH per liter of solution. The solution was pumped into the anode chambers of the four cells in series at a rate of 418 ml./hr. A catholyte was formulated and passed through the cathode compartments of the cells in series so that the equivalent of 0.4 mole/hour of H PO, and 0.50 mole/hour of KOH were passed through the cells. The water content of the catholyte was controlled so that the catholyte solution removed from the cells and the mother liquor recovered from the evaporator-crystallizer totaled about 418 ml./hr. The anolyte entering the first cell had a pH of about 12. Potassium hydroxide was added to the third cell of the series at a rate of about 0.23 mole/hour in order to keep the pH at about 12. After leaving the anode compartment, the anolyte was placed in an evaporator-crystallizer and water was evaporated until 80 percent of the potassium peroxydiphosphate in solution was precipitated and recovered. The mother liquor recovered from the evaporator-crystallizer and the catholyte issuing from the last cell were mixed together to give a total solution of 418 mL/hr. This was recycled to the anode compartment as recycle anolyte, continuously. The fluoride content of the recycle anolyte stream was continuously monitored and adjusted to 2.4 molar by adding HP. The current flow during electrolysis was 4.2 a. The production rate of potassium peroxydiphosphate during this portion of the run was 69.5 g./hour.

As the above system was operating in continuous cycle, conditions, particularly flow rates, were changed until the system reached equilibrium. At that point the recycle anolyte solution, which is a mixture of motor liquor from the crystallizer and catholyte from the cathode chamber, contained the equivalent of 0.12 mole K P O L76 moles H PO 2.4 moles KF and 4.93 moles KOH per liter and was recycled at a rate of 522 ml./hour. The cathode solution circulated the equivalent of 0.50 mole of l-l P0. per hour and 0.78 mole /hour of passed through the cathode compartments of the cells. Suffi cient water was added to the catholyte so that after mixing the cathode eri'luent with the mother liquor coming from the evaporator-crystallizer, yielding 86.8 g./hour of K3 0 When equilibrium was reached, the current efficiency in the above example was percent at an anode current density of 0.3 a./cm." The conversion of phosphate to peroxydiphosphate per pass through the anode cells was 70 percent, and the potassium peroxydiphosphate removal in the crystallizer per pass was 80 percent.

Pursuant to the requirements of the patent statutes, the principle of this invention has been explained and exemplified in a manner so that it can be readily practiced by those skilled in the art, such exemplification including what is considered to represent the best embodiment of the invention. However, it should be clearly understood that, within the scope of the appended claims, the invention may be practiced by those skilled in the art, and having the benefit of this disclosure otherwise than as specifically described and exemplified herein.

What is claimed is:

1. Process for producing potassium peroxydiphosphate comprising introducing aqueous mixture consisting essentially of potassium, phosphate and fluoride ions into the anode compartment of an electrolytic cell as the anolyte, said anolyte having a phosphate ion concentration of from about 1 to about 4 molar, a fluoride ion concentration of at least about 0.5 to 1.25 atoms of fluoride ion for each phosphate ion and a potassium ion concentration sufficient to have at least one potassium ion present for every fluoride ion and in addition from two to three potassium ions present for each phosphate ion in said anolyte solution, introducing an aqueous mixture consisting essentially of phosphate ions into the cathode compartment of said electrolytic cell as the catholyte, the catholyte and anolyte of said electrolytic cell being contained by means which permit ions to pass freely between the anolyte and catholyte but which prevent any substantial amounts of peroxydiphosphate values in the anolyte from mixing with the catholyte, passing an electric current through said catholyte and anolyte by means of a cathode in said catholyte and an anode in said anolyte, converting phosphate values to peroxydiphosphate values from said anolyte at a pH of from about 9.7 to about 14.5, removing an anolyte enriched in peroxydiphosphate values from said electrolytic cell, and precipitating and recovering potassium peroxydiphosphate values from said anolyte.

2. Process of claim 1 wherein the concentration of potassium ions in the catholyte is increased by the migration of potassium ions from said anolyte to said catholyte, said anolyte enriched in peroxydiphosphate values is removed from said electrolytic cell and concentrated to precipitate potassium peroxydiphosphate values, separating and recovering the precipitated potassium peroxydiphosphate values from the anolyte mother liquor, recycling said anolyte mother liquor to said anode compartment along with the potassium-enriched catholyte solution and introducing a fresh, aqueous mixture consisting essentially of phosphate ions into the cathode compartment as the catholyte.

3. Process of claim 2 wherein said anolyte solution has a pH of from 11.8 to 12.6 prior to precipitation of said potassium peroxydiphosphate therefrom.

4. Process of claim 1 wherein said anolyte has a phosphate ion concentration of from about 1 to about 4 molar, a fluoride concentration of from about 0.1 to about 5 molar and a potassium ion concentration sufficient to have one potassium ion present for every fluoride ion and in addition from two to three potassium ions present for each phosphate ion in said anolyte solution, and said catholyte is an aqueous mixture consisting essentially of potassium and phosphate values in a K/PO, molar ratio of 0.5-3:1.

5. Process of claim 1 wherein said anolyte has a phosphate ion concentration of from about 2 to about 3 molar, a fluoride concentration of from about 1 to about 316 m o1ar 51135 potassium ion concentration sufficient to have one potassium ion present for every fluoride ion and in addition from 2.5 to 2.8 potassium ions present for each phosphate ion in said anolyte solution, and said catholyte is an aqueous mixture consisting essentially of potassium and phosphate values in a K/PO molar ratio of about 2: 1.

6. Process of claim 1 wherein the ph of the anolyte is from about 11.8 to about 13.5

7. Process of claim 1 wherein the pH of the anolyte is from about 12.2 to about 12.6.

8. Process of claim 1 wherein potassium hydroxide is added to said anolyte to maintain its pH between 9.7 and 14.5 during electrolysis.

9. Process of claim 1 wherein the catholyte and anolyte are separated and contained by diaphragm means.

10. Continuous process for producing potassium peroxydiphosphate comprising introducing an aqueous mixture consisting essentially of potassium, phosphate and fluoride ions into the anode compartment of an electrolytic cell as the anolyte, said anolyte having a phosphate ion concentration of from about l to about 4 molar, a fluoride ion concentration of at least about 0.5 to 1.25 atoms of fluoride ion for each phosphate ion and a potassium ion concentration sufiicient to have at least one potassium ion present for every fluoride ion and in addition from two to three potassium ions present for each phosphate ion in said anolyte solution, introducing an aqueous potassium phosphate solution having a K/PO molar ratio of 0.5-3:1 into the cathode compartment of said electrolytic cell as the catholyte, the catholyte and anolyte of said electrolytic cell being contained by means which permit at least potassium ions to pass freely between the anolyte and catholyte but which prevent any substantial amounts of peroxydiphosphate values in the anolyte from mixing with the catholyte, passing an electric current through said catholyte and anolyte by means of a cathode in said catholyte and an anode in said anolyte, converting phosphate values to peroxydiphosphate values in said anolyte at a pH of from about 9.7 to about 14.5, continuously removing an anolyte enriched in peroxydiphosphate values from said electrolytic cell, maintaining the pH of said anolyte solution at from 11.8 to 12.6, concentrating the removed anolyte solution to precipitate potassium peroxydiphosphate values, separating and recovering the precipitated potassium peroxydiphosphate from the anolyte mother liquor, continuously removing catholyte from said electrolytic cell, recycling a mixture of said anolyte mother liquor and the removed catholyte to said anode compartment and constantly introducing fresh, aqueous potassium phosphate solution having a K/PO, molar ratio of 0.5-3:1 into the cathode compartment as the catholyte.

IF i I i l A v I Y r UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 3 5 D t d 10/26/71 Inventofll) Paul R. Mucenieks It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 22 "HPO" should be omitted.

Column 3, lines 27 & 28 "a./cm. should read amp/cm.

Columg 3, lige 7 4 "PO HPOf or HZP'OU" should read POM', HPOo OI HZPOI; I

Column I, line 57 "catolyte" should read -catholyte.

Column 5, line 67 "22" (second occurenoe) should read 28-.

Column 5, lines 7 4-75 "peroxydiphophate" should read --peroxydiphosphate--.

Column 6, line 2 "liquid" should read --liquor--.

Column 6, line &5 "electronic" should read --electric--.

Column 6, line 46 "undersired" should read --undesired-.

Column 6, line 49 "limited" should read -limiting- Column 6, line 53 "of" should be omitted.

UNITED STATES PATENT OFFICE Patent No. Dated Inventor) Paul R. Mucenieks It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 73 "of" 2nd 0001. should read Wes Column 7, line 16 "7" should read -7 hours-.

Column 7, line 70 "motor" should read -mother-.

Column 7, line 72 "K P O,l.76" should read -K;,P2O l.76--.

Column 8, line 27 "introducing aqueous" should read -.-introducing an aqueous.

Column 9, line 10 "ph" should read --pH-.

Signed and sealed this 22nd day of January 197L (SEAL) Attest:

EDWARD M.FLETCHER,JR. RENE D. TEGTMIEIYER Attesting Officer Acting Commissioner of Patents 

2. Process of claim 1 wherein the concentration of potassium ions in the catholyte is increased by the migration of potassium ions from said anolyte to said catholyte, said anolyte enriched in peroxydiphosphate values is removed from said electrolytic cell and concentrated to precipitate potassium peroxydiphosphate values, separating and recovering the precipitated potassium peroxydiphosphate values from the anolyte mother liquor, recycling said anolyte mother liquor to said anode compartment along with the potassium-enriched catholyte solution and introducing a fresh, aqueous mixture consisting essentially of phosphate ions into the cathode compartment as the catholyte.
 3. Process of claim 2 wherein said anolyte solution has a pH of from 11.8 to 12.6 prior to precipitation of said potassium peroxydiphosphate therefrom.
 4. Process of claim 1 wherein said anolyte has a phosphate ion concentration of from about 1 to about 4 molar, a fluoride concentration of from about 0.1 to about 5 molar and a potassium ion concentration sufficient to have one potassium ion present for every fluoride ion and in addition from two to three potassium ions present for each phosphate ion in said anolyte solution, and said catholyte is an aqueous mixture consisting essentially of potassium and phosphate values in a K/PO4 molar ratio of 0.5- 3:1.
 5. Process of claim 1 wherein said anolyte has a phosphate ion concentration of from about 2 to about 3 molar, a fluoride concentration of from about 1 to about 3.6 molar and a potassium ion concentration sufficient to have one potassium ion present for every fluoride ion and in addition from 2.5 to 2.8 potassium ions present for each phosphate ion in said anolyte solution, and said catholyte is an aqueous mixture consisting essentially of potassium and phosphate values in a K/PO4 molar ratio of about 2:
 6. Process of claim 1 wherein the ph of the anolyte is from about 11.8 to about 13.5
 7. Process of claim 1 wherein the pH of the anolyte is from about 12.2 to about 12.6.
 8. Process of claim 1 wherein potassium hydroxide is added to said anolyte to maintain its pH between 9.7 and 14.5 during electrolysis.
 9. Process of claim 1 wherein the catholyte and anolyte are separated and contained by diaphragm means.
 10. Continuous process for producing potassium peroxydiphosphate comprising introducing an aqueous mixture consisting essentially of potassium, phosphate and fluoride ions into the anode compartment of an electrolytic cell as the anolyte, said anolyte having a phosphate ion concentration of from about 1 to about 4 molar, a fluoride ion concentration of at least about 0.5 to 1.25 atoms of fluoride ion for each phosphate ion and a potassium ion concentration sufficient to have at least one potassium ion present for every fluoride ion and in addition from two to three potassium ions present for each phosphate ion in said anolyte solution, introducing an aqueous potassium phosphate solution having a K/PO4 molar ratio of 0.5- 3:1 into the cathode compartment of said electrolytic cell as the catholyte, the catholyte and anolyte of said electrolytic cell being contained by means which permit at least potassium ions to pass freely between the anolyte and catholyte but which prevent any substantial amounts of peroxydiphosphate values in the anolyte from mixing with the catholyte, passing an electric current through said catholyte and anolyte by means of a cathode in said catholyte and an anode in said anolyte, converting phosphate values to peroxydiphosphate values in said anolyte at a pH of from about 9.7 to about 14.5, continuously removing an anolyte enriched in peroxydiphosphate values from said electrolytic cell, maintaining the pH of said anolyte solution at from 11.8 to 12.6, concentrating the removed anolyte solution to precipitate potassium peroxydiphosphate values, separating and recovering the precipitated potassium peroxydiphosphate from the anolyte mother liquor, continuously removing catholyte from said electrolytic cell, recycling a mixture of said anolyte mother liquor and the removed catholyte to said anode compartment and constantly introducing fresh, aqueous potassium phosphate solution having a K/PO4 molar ratio of 0.5-3:1 into the cathode compartment as the catholyte. 